linux - Pthread Mutex lock unlock by different threads

Question

A Naive question ..

I read before saying - "A MUTEX has to be unlocked only by the thread that locked it."

But I have written a program where THREAD1 locks mutexVar and goes for a sleep. Then THREAD2 can directly unlock mutexVar do some operations and return.

==> I know everyone say why I am doing so ?? But my question is - Is this a right behaviour of MUTEX ??

==> Adding the sample code

void *functionC()
{
   pthread_mutex_lock( &mutex1 );
   counter++;
   sleep(10);
   printf("Thread01: Counter value: %d
",counter);
   pthread_mutex_unlock( &mutex1 );
}

void *functionD()
{
   pthread_mutex_unlock( &mutex1 );
   pthread_mutex_lock( &mutex1 );
   counter=10;
   printf("Counter value: %d
",counter);
}

int main()
{
   int rc1, rc2;
   pthread_t thread1, thread2;

   if(pthread_mutex_init(&mutex1, NULL))
   printf("Error while using pthread_mutex_init
");

   if( (rc1=pthread_create( &thread1, NULL, &functionC, NULL)) )
   {   
      printf("Thread creation failed: %d
", rc1);
   }   

   if( (rc2=pthread_create( &thread2, NULL, &functionD, NULL)) )
   {   
      printf("Thread creation failed: %d
", rc2);
   } 

Answer 1

Pthreads has 3 different kinds of mutexes: Fast mutex, recursive mutex, and error checking mutex. You used a fast mutex which, for performance reasons, will not check for this error. If you use the error checking mutex on Linux you will find you get the results you expect.

Below is a small hack of your program as an example and proof. It locks the mutex in main() and the unlock in the created thread will fail.

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>

/*** NOTE THE ATTR INITIALIZER HERE! ***/
pthread_mutex_t mutex1 = PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP;

int counter = 0;


void *functionD(void* data)
{
   int rc;

   if ((rc = pthread_mutex_unlock(&mutex1)) != 0)
   {
       errno = rc;
       perror("other thread unlock result");
       exit(1);
   }

   pthread_mutex_lock(&mutex1);
   counter=10;
   printf("Thread02: Counter value: %d
",counter);

   return(data);
}


int main(int argc, char *argv[])
{
   int rc1;
   pthread_t thread1;

   if ((rc1 = pthread_mutex_lock(&mutex1)) != 0)
   {
       errno = rc1;
       perror("main lock result");
   }

   if( (rc1 = pthread_create(&thread1, NULL, &functionD, NULL)))
   {
      printf("Thread creation failed: %d
", rc1);
   }

   pthread_join(thread1, NULL);
}